Solid base catalysts

Aminopropylsilyl-tethered mesoporous siUcates such as MCM-41 also catalyze the Knoevenagel condensation of benzaldehyde with ethyl cyanoacetate in toluene solvent [109]. Mesoporous alumina also acted as a base catalyst for the Knoevenagel condensation of benzaldehyde with ethyl cyanoacetate [110]. 

Fluorapatite, activated by water and benzyltriethylammonium chloride, resulted in a new solid catalyst for the Knoevenagel condensation of aldehydes with nitriles 

The COj species in the HT interlayer could be exchanged with OH ions by calcination at 723 K and hydration at room temperature. A spinel phase of Mg-Al mixed oxide obtained after the calcination transforms into the original layered structure during the hydration. This reconstruction is known as the memory effect of HT materials. The reconstructed HT catalyzed the Knoevenagel condensation of various aldehydes with nitriles in the presence of water [119]. The reconstracted HT also showed an aqueous Michael reaction of nitriles with a,p-unsaturated compounds. The layered double-hydroxide-supported diisopropylamine catalyzed the Knoevenagel condensation of aromatic carbonyl compounds with malononitrile or ethyl cyanoacetate [120]. This solid base could be recycled at least four times, and exhibited activity for aldol, Henry, Michael, transesterification, and epoxidation of alkenes. 

Highly thermally stable, three-dimensional, spongelike mesoporous Ce Zri 02 nanocrystallines acted as acid-base bifunctional solid solutions for the Knoevenagel condensation of aldehydes with active methylene compounds [44]. This catalyst can be recycled at least twice for the reaction of benzaldehyde with malononitrile. 

Previous sections have shown that catalysis by solid acids has received much attention due to its importance in petroleum refining and petrochemical processes. Conversely, relatively few studies have focused on catalysis by bases, even if acid and base are paired concepts. Base catalysts, however, play a decisive role in several reactions essential for fine-chemical syntheses [248-251]. Solid-base catalysts have many advantages over liquid bases. Examples of successfijl reactions include isomerization, aldol condensation, Knoevenagel condensation, Michael condensation, oxidation and Si—C bond formation. Various reviews have discussed catalysis by solid bases [248-255]. 

Single metal oxides Alkaline earth and alkali metal oxides (MgO, CaO, SrO, BaO) Rare earth oxides (La203, Yh02) Transition metal oxides (Th02, Zr02, ZnO, Ti02) 

Micro- and meso- Alkali ion-exchanged zeolites and mesoporous materials 

Supported alkali Alkali metal ions on alumina (KF/AI2O3, Na/NaOH/Al203) 

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BaO is used to impart improved strength to porcelain (34), as a solid base catalyst, in specialty cements, and for drying gases. 

The other main support used for solid base catalysts is polystyrene, which while it does not have a well-defined porous structure, does swell in solvents providing an accessible high surface area on which to carry out reactions. One common method of chemically attaching groups to polystyrene involves incorporation of specific amounts of styrene contain-... 

Examples of commercially applied solid base catalysts are much fewer than for solid acids. Nevertheless, much attention is currently focused on the development of novel solid base catalysts for classical organic reactions such as aldol condensations, Michael additions, and Knoevenagel condensations, to name but a few. 

The Aldol Condensation of Acetone Over a CsOH/Si02 Solid Base Catalyst... 

This paper investigates the acetone condensation reaction in the vapour phase over a CsOH/Si02 solid based catalyst over a range of reaction temperatures using hydrogen and deuterium as carrier gases. 

The solid base catalysts were prepared by dissolving Cs(N03)2 (Aldrich, 99%) in the minimum amount of distilled water before addition to the silica support by spray impregnation a method used to give a high dispersion of the metal salt on the support. The amount of each precursor added was calculated in order to give a 10% loading of metal on each catalyst. The catalyst was then dried in an oven overnight at 373 K. Prior to the reaction the catalyst was calcined in situ in a flow of N2 (BOC, 02 free N2) at 10 cm3 min"1 for 2 hours at 723 K. 

Friedel-Crafts reactions involving electrophilic substitution of aromatic compounds have been reported on solid base catalysts such as thallium oxide and MgO. The rates of benzylation of toluene by benzyl chloride over MgO nanocrystals were found to be of the order CP-MgO > CM-MgO > AP-MgO.56 An important observation in the study was that x-ray diffraction of the spent catalyst... 

Ono, Y. Baba, T. Selective reactions over solid base catalysts. Catal. Today 1997, 38, 321-337. 

Utiyama, M. Hattori, H. Tanabe, K. Exchange reaction of methane with deuterium over solid base catalysts. J. Catal. 1978, 53, 237-242. 

Solid base catalysts with a high conversion efficiency... 

Wieland, W.S., Davis, R.J., and Garces, J.M. (1996) Solid base catalysts for side-chain alkylation of toluene with methanol. Catal Today, 28, 443-450. 

It appears to us that the way forward to further industrial application of solid base catalysts may emerge from the discovery of unusual selectivity effects or new reactions that require well-balanced base and acid-base pair sites with relatively inexpensive solid catalysts that last long enough to process large quantities of reactants per unit mass of catalyst. 

Alkaline earth metal oxides have been used as solid base catalysts for a variety of organic transformations. Excellent reviews by Tanabe 4) and Hattori 2,3,7) provide detailed information about the catalytic behavior of alkaline earth metal oxides for several organic reactions of importance for industrial organic synthesis. In this section, we describe in detail reactions that have been reported recently to be catalyzed by alkaline earth metal oxides. 

The same authors (77) also investigated the Michael addition of nitromethane to a,/l-unsaturated carbonyl compounds such as methyl crotonate, 3-buten-2-one, 2-cyclohexen-l-one, and crotonaldehyde in the presence of various solid base catalysts (alumina-supported potassium fluoride and hydroxide, alkaline earth metal oxides, and lanthanum oxide). The reactions were carried out at 273 or 323 K the results show that SrO, BaO, and La203 exhibited practically no activity for any Michael additions, whereas MgO and CaO exhibited no activity for the reaction of methyl crotonate and 3-buten-2-one, but low activities for 2-cyclohexen-l-one and crotonaldehyde. The most active catalysts were KF/alumina and KOH/alumina for all of the Michael additions tested. 

Solid base catalysts were tested for this reaction, which was carried out in benzene as a solvent at 313 K. Among them, MgO, CaO, and SrO exhibited good catalytic properties, affording phthalide exclusively with yields between 86% and 100% after a short time (15min) in a batch reactor, whereas with y-alumina, phthalide was obtained with excellent yields and selectivity after 4h. The Tishchenko reaction of 2,3-naphthalenedicarbaldehyde was also carried out with CaO and with y-alumina at 333 K yields of the corresponding five-membered lactone were 94% and 100% after 2 and 20 h, respectively. 

The chapters in this volume illustrate how molecular concepts underlie catalysis. They illustrate how modern concepts of biology are influencing catalysis and catalyst discovery how concepts of homogeneous and surface catalysis have merged (a theme that is evident in the preceding several volumes of the Advances), exemplified by dendrimer catalysts that have properties of both molecules and surfaces and how concepts of molecular catalysis by bases have influenced the development of new solid-base catalysts and fundamental understanding of how they function. 

In addition to transesterification reactions, solid base catalysts, including both simple oxides and zeolitic materials, have been used to carry out esterification reactions of fatty acids. These studies have mainly focused on the... 

The results in Table 2 show that the pyridine is less active than any of the X zeolites and Ge faujasite except the lithium form which shows slightly lower activity, whereas all Y zeolites show lower activity than pyridine. Piperidine, however, is more active than any of the zeolite samples studied here. From this comparison, it appears that, most of the basic sites of the zeolites must have pK<10.3. However, the fact that zeolites are also active for catalyzing the condensation of benzaldehyde with ethyl malonate, indicate that these samples have some basic sites with pK< 13.3. On a quantitative bases, and comparing the activity of zeolites for condensation with ethyl cyanoacetate, ethyl acetoacetate and ethyl malonate (Fig. 2), we can conclude that most of the basic sites of the zeolite have pK<9.0 with a sensible amount with 9.0basic strength of different solid base catalysts. 

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